Receiver, Communication Apparatus, Method and Computer Program for Receiving an Amplitude Shift Keyed Signal

ABSTRACT

A method of a receiver is used for receiving an amplitude shift keyed signal provided over a multi-layered transmission from a plurality of antennas with different precoding of different symbols for the respective layers. The method comprises receiving a sequence of signal values of the signal, estimating, from the sequence of signal values, channels for the respective layers, and selecting one of a plurality of detection methods based on a difference in quality between the estimated channels A receiver and a computer program are also disclosed.

The project leading to this application has received funding from theEuropean Union's Horizon 2020 research and innovation programme undergrant agreement No 641985.

TECHNICAL FIELD

The present disclosure generally relates to a receiver, a communicationapparatus, methods therefor, and computer programs for implementing themethods. In particular, the disclosure relates to receiving a wirelesssignal carrying binary information in a way less prone to fading.

BACKGROUND

The telecommunications domain has often so forth been accompanied by asignificant increase of electrical energy consumption. Demands onperformance, such as spectral efficiency or data rate, have been met atthe expense of more energy consumption. Advances in analogue and digitalelectronics have enabled development of low-cost, low-energy wirelessnodes. However, energy consumption remains an issue for someapplications. The approach used for idle mode listening, especially whenused by devices related to the field commonly referred to as Internet ofThings, IoT, in wireless networks impacts the overall energy consumptionfor the devices. This is particularly noticeable when the data trafficis very sporadic.

Energy reduction may for example be performed by an approach in which itis possible to switch off a radio frequency main interface duringinactive periods and to switch it on only if a communication demandoccurs. For example, by using a wake-up radio, where a wake-up signal issent by using a transmitter, received and decoded at the device, whereinthe main radio is activated, significant energy consumption reductionmay be achieved for many applications.

Furthermore, efforts to reduce energy consumption may be made atdifferent levels of the communication stack, such as the medium accesscontrol (MAC) protocol, by dynamically adapting the sleep and wake timesof main radio protocols. Limited complexity signals and thus limitedcomplexity decoders for the intermittently presented control signals mayimprove energy efficiency.

These efforts affect the physical layer (PHY), where control mechanismsfor activation or deactivation of more energy consuming operationsreside, which put demands on lean control signalling.

An example in the PHY is application of an On-Off Keying, OOK, signal asillustrated in FIG. 1, which is a modulation scheme where the presenceof a signal represents the ON part or state and the absence of thesignal represents the OFF part or state. For example, the ON and OFFparts could represent binary digits. OOK is considered the simplest formof amplitude-shift keying, ASK, that represents digital data at thepresence or absence of a signal. In its simplest form, the presence of acarrier for a specific duration represents a binary one, while itsabsence for the same duration represents a binary zero. Some moresophisticated schemes vary these durations to convey additionalinformation. OOK is analogous to unipolar encoding, which is a specialcase of a line code. OOK is a suitable modulation to use whenever thepower consumption of the receiver is a major concern, as thedemodulation can be done non-coherently, with very relaxed requirementson gain control and resolution in the receiver.

In order to decode OOK, the receiver has to estimate which signal levelcorresponds to the presence of a signal and which signal levelcorresponds to the absence of a signal. Manchester Coding is amodulation means where the transition between ON to OFF state and OFF toON state could represent binary digits, and may be used to simplifyclock recovery and to simplify demodulation by ensuring that the averagesignal level of the signal carries no information. FIG. 2 illustrates adata bit with value one is represented by, i.e. encoded to, a logicalone followed by a logical zero, whereas a data bit with value zero isrepresented by a logical zero followed by a logical one. Alternatively,the encoding can be swapped so that a data bit with value one isrepresented by a logical zero followed by a logical one, etc.

Clock recovery is simplified because there will always be a transitionfrom zero to one or vice versa in the middle of each symbolirrespectively of what the data is.

The decoding of the Manchester coded symbol is essentially done bycomparing the first and the second half of the symbols and deciding infavour of a logical one if the first half of the symbol has largerenergy than the second half of the same symbol, or vice versa. Insteadof energy, one can also use other means of measuring the signal level,for example absolute signal-envelope averaged over the symbol duration.

For example, Manchester coded OOK is being standardized within the IEEE802.11ba task group (TG). TG 802.11ba develops a standard for wake-upradios (WUR), targeting to significantly reduce the power consumption indevices based on the 802.11 standard. It is proposed to generate thewake-up signal (WUS) by using an inverse fast Fourier transform (IFFT),as this block is already available in Wi-Fi transmitters supporting e.g.802.11a/g/n/ac. Specifically, an approach discussed for generating theOOK is to use the 13 sub-carriers in the centre, possibly excluding theDC carrier, and then populating these with some signal to represent ONand to not transmit anything at all to represent OFF.

IEEE document IEEE 802.11-17/0188r10, with title “IEEE 802.11 TGbaSimulation Scenarios and Evaluation Methodology Document” by ShahrnazAzizi et al, defines simulation scenarios, evaluation criteria andmethodology to be used for evaluation of performance of features andgeneration of simulation results.

SUMMARY

The disclosure is based on the inventors' understanding that receptionof a multi-layered signal with large differences in channel qualitiesbetween layers may be more efficiently decoded under some circumstances.

According to a first aspect, there is provided a method of a receiverfor receiving an amplitude shift keyed signal provided over amulti-layered transmission from a plurality of antennas with differentprecoding of different symbols for the respective layers. The methodcomprises receiving a sequence of signal values of the signal,estimating, from the sequence of signal values, channels for therespective layers, and selecting one of a plurality of detection methodsbased on a difference in quality between the estimated channels.

The receiving may comprise estimating signal values as energies of thesignal by an envelope detector, and the estimating of the channels maycomprise estimating channel gain by selecting at least one signal valueassociated with respective layer and performing the estimation of thechannels from the selected at least one signal value for respectivelayer. The estimating of the channels may comprise estimating phasedifferences between the channels.

The signal may be sent as a repeated symbol sequence for each layer, andthe selecting of the detection method may comprise determining whetherone of the estimated channels has a significantly lower quality thananother of the estimated channels. If the one of the estimated channelshas the significantly lower quality, the selected detection method maycomprise omitting a part of the signal with the lower quality anddecoding the rest of the signal. If there is no estimated channel havingthe significantly lower quality, the selected detection method maycomprise decoding the whole signal. The determining whether the one ofthe estimated channels has significantly lower quality than the anotherof the estimated channels may comprise determining whether differencebetween channel gains is greater than a first threshold.

The selecting of the detection method may comprise determining whetherone of the estimated channels has a significantly lower quality thananother of the estimated channels, and determining whether a largest ofchannel gain of one of the estimated channels and another of theestimated channels is greater than a second threshold. If the one of theestimated channels has the significantly lower quality and the largestchannel gain of the channel gains is greater than the second threshold,the selected detection method may comprise omitting a part of the signalwith the lower quality and decoding the rest of the signal. Otherwise,the selected detection method may comprise decoding the whole signal.

The signal may be sent as a repeated symbol sequence for each layer, andthe selecting of the detection method may comprise determining whether adifference between channel gain of one of the estimated channels andanother of the estimated channels is greater than a first threshold, anddetermining whether a largest of channel gain of one of the estimatedchannels and another of the estimated channels is greater than a secondthreshold. If the one of the estimated channels has the difference inchannel gains greater than the first threshold and the largest channelgain is greater than the second threshold, the selected detection methodmay comprise omitting a part of the signal with the lower quality anddecoding the rest of the signal. Otherwise, the selected detectionmethod may comprise decoding the whole signal.

According to a second aspect, there is provided a computer programcomprising instructions which, when executed on a processor of areceiver, causes the receiver to perform the method according to thefirst aspect.

According to a third aspect, there is provided a receiver arranged toreceive an amplitude shift keyed signal provided over a multi-layeredtransmission with different precoding for the respective layers. Thereceiver is arranged to receive a sequence of signal values of thesignal, estimate, from the sequence of signal values, channels for therespective layers, and select one of a plurality of detection methodsbased on a difference in quality between the estimated channels.

The receiver may comprise an envelope detector arranged to estimatesignal values as energies of the signal, wherein the estimation of thechannels may comprise estimation of channel gain by selecting at leastone signal value associated with respective layer and performing theestimation of the channels from the selected at least one signal valuefor respective layer. The estimation of the channels may compriseestimation of phase differences between the channels.

The signal may be sent as a repeated symbol sequence for each layer,wherein to select the detection method the receiver may be arranged todetermine whether one of the estimated channels has a significantlylower quality than another of the estimated channels. If the one of theestimated channels has the significantly lower quality, the selecteddetection method may comprise omitting a part of the signal with thelower quality and decoding the rest of the signal. If there is noestimated channel having the significantly lower quality, the selecteddetection method may comprise decoding the whole signal. Thedetermination whether the one of the estimated channels hassignificantly lower quality than the another of the estimated channelsmay comprise a determination whether difference between channel gains isgreater than a first threshold.

The receiver may be arranged to, for the selection of the detectionmethod, determine whether one of the estimated channels has asignificantly lower quality than another of the estimated channels, anddetermine whether a largest of channel gain of one of the estimatedchannels and another of the estimated channels is greater than a secondthreshold. If the one of the estimated channels has the significantlylower quality and the largest channel gain of the channel gains isgreater than the second threshold, the selected detection method maycomprise omitting a part of the signal with the lower quality anddecoding the rest of the signal. Otherwise, the selected detectionmethod may comprise decoding the whole signal.

The signal may be sent as a repeated symbol sequence for each layer,wherein the receiver may be arranged to, for the selection of thedetection method, determine whether a difference between channel gain ofone of the estimated channels and another of the estimated channels isgreater than a first threshold, and determine whether a largest ofchannel gain of one of the estimated channels and another of theestimated channels is greater than a second threshold. If the one of theestimated channels has the difference in channel gains greater than thefirst threshold and the largest channel gain is greater than the secondthreshold, the selected detection method may comprise omitting a part ofthe signal with the lower quality and decoding the rest of the signal.Otherwise, the selected detection method may comprise decoding the wholesignal.

The receiver may be arranged to operate as a wake-up receiver arrangedto control on and off states of a main transceiver, which is co-locatedor integrated with the receiver, based on the signal received by thereceiver.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as additional objects, features and advantages of thepresent disclosure, will be better understood through the followingillustrative and non-limiting detailed description of preferredembodiments of the present disclosure, with reference to the appendeddrawings.

FIG. 1 is a signal diagram schematically illustrating an on-off keyingsignal.

FIG. 2 is a signal diagram which illustrates a data bit with valuerepresentation.

FIG. 3 is a signal diagram schematically illustrating signals propagatedthrough different channels obtained by using different precoding.

FIG. 4 is a signal diagram schematically illustrating a signal obtainedby using varying precoding.

FIG. 5 is a flow chart schematically illustrating a transmitter methodaccording to an example.

FIG. 6 is a flow chart schematically illustrating a transmitter methodaccording to an example.

FIG. 7 is a flow chart schematically illustrating a receiver methodaccording to an embodiment.

FIG. 8 is a flow chart schematically illustrating a receiver methodaccording to an embodiment.

FIG. 9 schematically illustrates a transmitter according to an example.

FIG. 10 schematically illustrates a receiver according to an embodiment.

FIG. 11 is a block diagram schematically illustrating a communicationdevice according to an embodiment.

FIG. 12 schematically illustrates a computer-readable medium and aprocessing device.

FIG. 13 is a graph showing packet error rate (PER) for some embodiments.

FIG. 14 is a graph showing packet error rate (PER) for some embodiments.

DETAILED DESCRIPTION

Wake-up receivers (WUR), sometimes also referred to as wake-up radios,provide a means to significantly reduce the power consumption inreceivers used in wireless communication. The idea with a WUR is that itcan be based on a very relaxed architecture, as it only needs to be ableto detect the presence of a wake-up signal, but will not be used for anydata reception.

A commonly used modulation for the wake-up packet (WUP), i.e., thesignal sent to the WUR, is on-off keying (OOK). OOK is a binarymodulation amplitude shift keyed approach, where a logical one isrepresented with sending a signal (ON) whereas a logical zero isrepresented by not sending a signal (OFF), or vice versa.

There are currently activities ongoing in the IEEE 802.11 task group(TG) named IEEE 802.11ba to standardize the physical (PHY) and mediumaccess control (MAC) layers for a Wake-Up Radio to be used as acompanion radio to the 802.11 primary communications radio (PCR) withthe mere purpose to significantly reduce the power consumption.

It is proposed to generate the wake-up signal (WUS) by using an inversefast Fourier transform (IFFT), as this block is already available inWi-Fi transmitters supporting e.g. IEEE 802.11a/g/n/ac. Specifically, anapproach discussed for generating the OOK is to use the 13 sub-carriersin the centre, and then populating these with some signal to representON and to not transmit anything at all to represent OFF. The IFFTtypically has 64 points and is operating at a sampling rate of 20 MHz,and just as for ordinary orthogonal frequency division multiplexing(OFDM) a cyclic prefix (CP) is added after the IFFT operation in orderto keep the OFDM symbol duration used in 802.11a/g/n/ac and thus be ableto spoof legacy stations by prepending a legacy preamble at thebeginning of the WUS. In this way legacy stations will be able to detectthe WUS and correctly defer access to the wireless medium.

To further ease the reception of the WUP, the wake-up signals (WUSs) areManchester-coded. That is the transmission of a logical zero” is done bysending OFF followed by ON, while the transmission of a logical one isdone by sending ON followed by OFF. The assigning of ON OFF and OFF ONpatterns to bit values may equally be the opposite. Depending on therequired data rate, one can either send one such ON/OFF-sequence orrepeat the ON/OFF-sequence multiple times. Repeating it multiple timeeffectively corresponds to using a repetition code. That is, a secondpart of the signal, e.g. symbol or bit representation, is a repeatedreplica of a first part of the signal.

When a WUS is transmitted over a wireless channel, the amplitude of thereceived signal is determined by the channel fading. The main benefit ofusing on-off keying, compared to coherent modulation formats, is thatthe pattern of ON/OFF transmissions can then be detected non-coherentlyby an envelope detector.

Fading of the wireless channel is an unavoidable practical limitation.If the channel is in a deep fade, then the performance, in sense ofrange from a practical point of view, of the WUS can be severelyreduced. In the above-mentioned IEEE 802.11ba standard, the goal is thatthe WUR should have the same range as the PCR. If the WUS is moresensitive to fading, e.g. due to that the signal has a smallerbandwidth, this means that the sensitivity may need to be improved inorder to allow for a larger fading margin.

The requirement of an improved sensitivity translates into an increasedcost as well as an increased power consumption of the WUR, thus reducingthe benefit of using a WUR. The suggested approach of this discloserrelies on the assumption that the transmitter of the WUS typically isequipped with multiple antennas, and that neither power consumption norcost is as critical for the transmitter of the WUS as it is for the WUR.

Specifically, several ways to achieve transmit diversity are disclosedwhere the reception can be performed in a cost and power efficient wayby a simple WUR.

In some examples the diversity scheme can be seen as achieving anantenna selection diversity gain, but without the need for thetransmitter to know which one of the antennas being selected by thereceiver.

Some of the examples are especially tailored for Manchester-codedtransmission, whereas other embodiments are applicable whether plain OOKor Manchester-coded OOK being used.

The suggested approach allows for improved link performance through anefficient and low complex introduction of transmit diversity. Theimproved link performance translates into enhanced coverage and reducedpower consumption. The disclosed approach can also be used to transmitadditional information.

The approach above may be used for lean or extremely lean transmissions,such as for wake-up signal to a wake-up radio in a receiver, where thewake-up radio has the purpose of receiving the wake-up signal and uponproper decoding thereof initiate operation of a main transceiver of thereceiving entity, wherein the main transceiver commences trafficexchange with e.g. a network node. Here, the network node may be theentity comprising the transmitter discussed above. Features of receiversof such lean or extremely lean transmissions are often that they are lowcomplexity and low power consuming. This normally leads to that they arespecified for low bitrate communication. An example is that they arearranged to operate with a bitrate of 1/100 to 1/1000 of what isnormally or in feasible operation modes used on a channel between thenetwork node and the receiving entity, in view of the wake-up signalbitrate to bitrate of PCR signal providing for the extremely leantransmissions for the wake-up signal.

A WUS is transmitted from a transmitter with at least two antennas to areceiving user. For brevity, we exemplify the operation in the case oftwo transmit antennas. Alternatively, the notation of “antenna 1” and“antenna 2” can be interpreted as virtual antennas created by twodifferent precoding vectors, which are transmitted using more than twoantennas. Denote the channel response from antenna 1 to the user by h₁and the channel response from antenna 2 by h₂.

The two antennas generally transmit different signals. In thedescription below, the total transmitted power, denoted p, can bearbitrarily divided between the two transmit antennas. In case there areother constraints on the transmitted power, e.g. that the average powerfrom each antenna is limited, the principle described below may have tobe slightly modified. However, since such modification should bepossible to do for a person of ordinary skill in the art, the inventionis described for the case that the power limitation refers to the totalpower of the two transmit antennas in order to simplify the descriptionof the basic idea.

The different embodiments will also be described for the case thatManchester coded OOK is used. It is also discussed, for the differentembodiments, if the corresponding approaches would be applicable alsofor plain OOK.

The phase difference between the antennas can also be varied. Thisvariation of phase and power per antenna may be viewed as “precoding”.The phase-amplitude differences between the transmitted signals inducedby the precoding may be expressed as a “precoding matrix” W where eachrow corresponds to an antenna and each column corresponds to a timeindex, or vice versa depending on implementation choice. Herein, thegeneral term “line” will be used for row or column of the matrix.

For example, suppose the precoding is performed with the followingmatrix:

$W = {\sqrt{p}\begin{bmatrix}1 & 0 & \frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}} & \frac{\sqrt{3}}{2} & \frac{\sqrt{3}}{2} \\0 & 1 & \frac{1}{\sqrt{2}} & {- \frac{1}{\sqrt{2}}} & \frac{1}{2} & {- \frac{1}{2}}\end{bmatrix}}$

The first column [10]^(T) represents that, at the first time instant,transmission takes place only through antenna 1, with full power andzero phase-shift. The second column [01]^(T) represents that, at thesecond time instant, all power is transmitted through the second antennawith zero phase-shift. The third column [1/√21/√2]^(T) represents that,at the third instant, equal power is transmitted through both antennas,with the same phase. The fourth column [1/√2 −1/√2]^(T) represents that,at the fourth time instant, equal power is transmitted through the twoantennas, but with a relative phase difference of 180 degrees. Thecolumns of the matrix hence have the role of “precoding vectors”.

FIG. 3 is a signal diagram schematically illustrating signals propagatedthrough different channels obtained by using different precoding. In theexample above, if the first column of W was used constantly at all timeinstances, the received WUS would look like the upper graph of FIG. 3(where 0 corresponds to OFF and 1 corresponds to ON). Similarly, if thesecond column was used at all time instances, the received WUS wouldlook like the lower graph of FIG. 3. In the example, the second antennahas a stronger channel, |h₂|>|h₁|. That is, the signal received byantenna 2 contains the same transitions as the signal received byantenna 1, but the received amplitude is higher when transmitting an ON.

FIG. 4 is a signal diagram schematically illustrating a signal obtainedby using varying precoding. By varying the precoding, i.e., the choiceof vectors from the precoding matrix W, between each transmission of twoconsecutive ON-OFF symbols, the received signal may look as illustratedin FIG. 4. Each choice of precoding vector results in a particularreceived amplitude of the ON transmissions. While transmission from onlyone of the antennas leads to a constant received amplitude associatedwith “ON”, variation of the precoding results in amplitude variations.These variations can be exploited by the receiver to achieve moreefficient transmission.

In the first embodiment, the receiver estimates the magnitudes of thechannel responses based on the reception of a predefined “pilot pattern”of precoding vectors. For example, if the first two columns in W (as itwas defined in the above example) are designated as pilots, the receivedsignals will be, assuming an envelope detector,

y ₁=|√{square root over (p)}h ₁ +n ₁|

when using the first column and

y ₂=|√{square root over (p)}h ₂ +n ₂|

when using the second column, where n₁ and n₂ represent additive noise.Based on these observations, the amplitudes of the channels can beestimated. For example, a simple estimate of |h₁| is y₁/√{square rootover (p)} and a simple estimate of |h₂| is y₂/√{square root over (p)}.These estimators give perfect estimates when the noise is negligible.Other more sophisticated estimators can be utilized as well.

As a continuation of the example, if the first three columns in W areutilized as pilots, the additional received (envelope-detected) signalfrom using the third column is

$y_{3} = \left| {{\sqrt{\frac{p}{2}}\left( {h_{1} + h_{2}} \right)} + n_{3}} \right|$

where n₃ represents additive noise. By using y₁, y₂, y₃, the receivercan not only estimate |h₁| and |h₂|, but also the phase differencesRe{h₁h₂*} and Im{h₁h₂*} between the two channels:

$\left. {{\left. {Re} \right\} = \frac{y_{1} + y_{2} - {2y_{3}}}{2\sqrt{p}}}\left( {Im} \right\}} \right)^{2} = {\frac{y_{1}y_{2}}{2p} - {\frac{y_{1} + y_{2} - {2y_{3}}}{2\sqrt{p}}.}}$

These estimates are perfect estimates when the noise level isnegligible. Other more sophisticated estimators can be utilized as well.For instance, by using a fourth column of W as an additional pilot, thesign of Im{h₁h₂*} can also be acquired. Note that only the phasedifference between the channels can be estimated (but not the absolutechannel phase) since the envelope detector only measures receivedenergy.

A receiver that has estimated the channels can use these estimates todetect which precoding vector that was used for transmitting aparticular Manchester-coded symbol.

According to a first example, the first few symbols may be transmittedwith predefined “pilot” precoding vectors to enable channel estimation.The remaining symbols are transmitted with unknown precoding vectors,where the precoding pattern is selected as a function of someinformation to be conveyed. This information may, for example, representdata or parity bits from a channel code.

It should here be understood that explicit pilots mean some knownsymbols. These known symbols may either be send solely for the purposeof estimating the channel, but it can also be so that the channel isestimated from a known sequence used for synchronization, such as asynchronization word. In this case, the synchronization word would beknown and effectively re-used for channel estimation once it has beenused to obtain synchronization.

When there are explicit pilots sent at the beginning of the packet, thisidea to transmit additional information works also when the Manchestercoding is not used. However, the use of Manchester coding may still bebeneficial e.g. to allow for a low complex receiver.

It should here be noted that the transmitter does not have any knowledgeabout which is the best column in the pre-coding matrix, since thechannels h₁ and h₂ are unknown for the transmitter. If this knowledgewould have been available at the transmitter, a suitable pre-codingvector could have been selected.

At the receiver, according to one embodiment, the pilots are used todetermine e.g. the expect received amplitude corresponding to thedifferent pre-coding vectors, i.e., corresponding to the differentcolumns in the pre-coding matrix W. If noise is neglected, the envelopewill only be non-zero when the transmitted signal is ON. How this isexploited if further illustrated in connection to the next embodiment.

In a second example, the information can be viewed as conveyed usingdifferential modulation, without relying on predefined “pilot” precodingvectors for channel estimation. The differential modulation is hereimplemented by means of Manchester coding. Specifically, an informationbit “1” may be transmitted by sending OFF-ON, and an information bit “0”may be transmitted by sending OFF-ON, or vice versa. This may be viewedas differential modulation since effectively the information istransmitted in the difference between the first and second half of thesymbol.

In one example, a low data rate is used such that each Manchester-codedsymbol is transmitted by sending two repeated ON-OFF sequences (i.e., ONOFF ON OFF or OFF ON OFF ON). For each symbol transmission, the firstON/OFF sequence is transmitted with a first, predetermined precoding andthe second ON/OFF sequence is transmitted with a second, predeterminedprecoding. In addition to detecting transitions in the Manchester code,the amplitude level will be different in the first half and the secondhalf of the symbol transmission. This difference in amplitude providesside-information to the symbol detector.

For example, if the first two columns of W are used as precoding vectorsfor the first and second half of a symbol, respectively, then theamplitude side-information is particularly useful when either |h₂|»|h₁|or |h₁|»|h₂|, which typically occurs when one of the channels is in deepfade and the other is not. The detector may then first estimate |h₁| and|h₂|, based on two transmitted pilots, as described above, or bymeasuring the average received power in the first and second half of asymbol transmission. In case |h₁|»|h₂|, the detector only uses the firsthalf of each symbol transmission for detection. In case |h₂|»|h₁|, thedetector only uses the second half of each symbol transmission fordetection. If none of these cases applies, i.e. |h₁| and |h₂| areapproximately equally large, then a conventional detection mode may beused.

FIG. 5 is a flow chart schematically illustrating methods of atransmitter according to examples demonstrated above. The method is fortransmitting an amplitude shift keyed signal provided over amulti-layered transmission with different precoding for the respectivelayers. Here, the term “layers” refers to the use of multiple antennastransmitting modified versions of a signal, i.e. the signal is givendifferent precodings. The method comprises obtaining 500 a sequence ofbits to be conveyed. The sequence is keyed 502, i.e. the bits get arepresentation, e.g. by Manchester coding, to form the signal. Thesignal is then precoded 504 to respective layer, and transmitted 506using two or more of the antennas.

In some examples, additional side-information regarding a user, e.g.,regarding previously successful transmission of a WUS, spatialdirections, or other channel statistics, may be utilized for selecting503 a precoding for said user. This information can also be used whenselecting 503 precoding in view of other users, e.g. to avoidinterference.

This example of a scheme that utilizes two transmit antennas to achieveantenna diversity when the receiver is equipped with an envelopedetector and each Manchester-coded symbol is transmitted using multipleON-OFF sequences.

It can also be noted that it would be possible to decode this signal fora “legacy receiver”, which would simply decode in a conventional wayirrespective of the relation between |h₁| and |h₂|.

In another embodiment, a higher data rate is used such that everyManchester-coded symbol is transmitted using only one ON-OFF sequence.In that case, every Manchester-coded “0” is transmitted with a first,predetermined precoding and every Manchester coded “1” is transmittedwith a second, predetermined precoding. In addition to detectingtransitions in the Manchester code, the amplitude level will bedifferent when transmitting a “0” as compared to when transmitting a“1”. This difference in amplitude provides side-information to thesymbol detector. For example, if the first two columns of W are used asprecoding vectors for “0” respectively “1”, then the amplitudeside-information is particularly useful when either |h₂|»|h₁| or|h₁|»|h₂|, which typically occurs when one of the channels is in deepfade and the other is not.

In more detail, as exemplification, suppose the precoding is selectedsuch that all “0” symbols are transmitted through antenna 1 and all “1”symbols are transmitted through antenna 2. The detector then firstestimates |h₁| and |h₂|, based on two transmitted pilots, as describedabove, or by measuring the average power received in the first andsecond half of a symbol transmission. In case |h₂|»|h₁|, the detectorcan deduce when a “1” was transmitted (assuming the signal-to-noiseratio is sufficient), since in that case the amplitude differencebetween ON and OFF is particularly large. Specifically, if a “1” istransmitted as OFF ON, then the detector can measure the received signalamplitude in the second half of the symbol interval. If it is largerthan a threshold, which is, e.g., determined by the noise variance, thedetector concludes that a “1” was transmitted. Otherwise, it concludesthat a “0”, transmitted as ON OFF, was transmitted. The oppositeprocedure is applied in case of |h₁|»|h₂|. This way, antenna diversityis achieved.

FIG. 6 is a flow chart schematically illustrating methods of atransmitter according to the examples demonstrated above. The method isfor transmitting an amplitude shift keyed signal provided over amulti-layered transmission with different precoding for the respectivelayers. Here, the term “layers” refers to the use of multiple antennastransmitting modified versions of a signal, i.e. the signal is givendifferent precodings. The method comprises obtaining 600 a sequence ofbits to be conveyed. The sequence is keyed 602, i.e. the bits get arepresentation, e.g. by Manchester coding, to form the signal. Thesignal is then divided 604 based on bit value, resulting in what may beseen as sub-signals. The respective sub-signal is then precoded 606,606′ to respective layer, and transmitted 608 using two or more of theantennas.

In some embodiments, additional side-information regarding a user, e.g.,regarding previously successful transmission of a WUS, spatialdirections, or other channel statistics, may be utilized for selecting603 a precoding for said user. This information can also be used whenselecting 603 precoding in view of other users, e.g. to avoidinterference.

This example of a scheme that utilizes two transmit antennas to achieveantenna diversity when the receiver is equipped with an envelopedetector and each Manchester-coded symbol is transmitted using multipleON-OFF sequences.

It can also be noted that it would be possible to decode this signal fora “legacy receiver”, which would simply decode in a conventional wayirrespective of the relation between |h₁| and |h₁|. The diversity gainis however not achieved by the legacy receiver. For illustrating thebenefits, examples, related to receiver end, are given below for theeasier understanding of the approaches.

FIG. 7 is a flow chart schematically illustrating methods of a receiveraccording to embodiments. The method comprises receiving 700 a signalrepresenting a sequence of bits keyed and transmitted e.g. asdemonstrated above. That is, the signal is multi-layered, wherein eachlayer may, due to the transmit diversity, have a channel which differsfrom the other layers. The channels are estimated 702 for the respectivelayers. It can then be determined 704 whether one of the channels havesignificantly lower channel quality. For example, channel gain such as|h₁|, |h₂|, etc. may be considered as a quality metric. A determinedquality metric for the respective channels may be compared an acomparison result may in turn be compared with a threshold definingwhether one channel has significantly lower quality. If no channel hassignificantly lower quality, the received signal is detected 710 usinglegacy procedures, i.e. performing detection on the whole receivedsignal. However, if one channel has significantly lower quality, thesignal part corresponding to the lower quality layer may be omitted 706and detection 708 is performed on the remaining part of the receivedsignal.

When different information, e.g. logical ones and zeros in case ofbinary information, is transmitted using different precoders, i.e., theactual selection of precoder is based on the information to betransmitted, there are different options for demodulating thisinformation. What option is to be used may for instance be based onconsidering the trade-off between performance and complexity. Toillustrate this, suppose binary information is transmitted using twoantennas, and where one antenna is used for transmitting a logical zero,whereas the other antenna is used for transmitting a logical one.

At the receiver side, the goal is to determine whether a logical zero ora logical one was transmitted. The information is clearly transmittedover two different channels, where the first channel is from the firstantenna at the transmitter to the antenna at the receiver, whereas thesecond channel is from the second antenna at the transmitter to thereceiver. Typically, the two channels can be assumed to experienceindependent fading so that typically the channel quality of the twochannels would be different and potentially very different. Intuitively,to obtain good receiver performance, one would like to use the better ofthe channels to a higher degree than worse of the two.

Let h₀ and h₁ denote the two channels and suppose a logical zero istransmitter over the channel h₀ and a logical one is transmitted overthe channel h₁. Furthermore, without loss of generality, suppose h₁ isthe better of the two channels. Specifically, suppose them magnitudes ofh₁ is larger than the magnitude of h₀, i.e., |h₁|>|h₀|. Now, suppose thebinary information is transmitted using the signal s₁ and a logical zerois transmitted using the signal s₀. The task for the receiver is toestimate whether s₁ or so has been sent. With |h₁|>|h₀| it is clear thatthe received signal will have better quality if s₁ is transmittedcompared to if so is transmitted, and therefore one may detect the so asbeing present as soon as s₁ cannot be detected. Viewed in this way, thequality of the decision of so being sent is effectively determined bythe quality of h₁, rather than h₀, and thus the receiver performancewill be determined by the best channel the logical zero in this examplewas transmitted over h₀ rather than h₁.

To make the example a bit more specific, suppose on-off keying (OOK) isused. In addition, suppose the data is Manchester code such that alogical one is represented by sending ON-OFF and a logical zero isrepresented by sending OFF-ON, respectively. A receiver for Manchestercoded OOK makes a decision by comparing the first half of the signalwith the second half of the signal. If the first part is larger, e.g.using some suitable metric like e.g. energy, than the second part, adecision is made in favour of a logical one being transmitted, since itis more likely that the actual sent signal was ON-OFF rather thanOFF-ON. Clearly, if the OFF-ON is sent over the channel h₀ and ON-OFF issent over the channel h₁, and comparing whether the first or second partof the signal contain most energy will not be a good approach if h₁ andh₀ are very different. Specifically, if h₀ is very small, the receivedpower will be very small when s₀ is transmitted and thus the probabilityof making an error will approach 50% in case s₀ is transmitted. Thecorresponding error probability when s₁ is transmitted may, on the otherhand, be very small. To address this issue, the receiver can insteadmodify how a decision is made and only use the standard approach when|h₀| and |h₁| are of approximately the same size. When one of thechannels is much better than the other, the receiver instead bases itsdecision on whether the signal transmitted over the better of the twochannels is present or not. Specifically if h₁ is found to be muchbetter than h₀, and in case s₁ being transmitted it is estimated thatthe energy in the first half of the signal minus the energy in thesecond half of the signal should be E₁, then the receiver may decidethat s₁ was transmitted in case the energy of the first half minus thesecond half exceeds E₁/2, i.e., half the expected energy. Effectively,the decision boundary for making a decision in favour of s₁ being senthas been shifted from zero to E₁/2 as an effect of that one is notexplicitly looking for the signal s₀ but indirectly consider s₀ as beingpresent if s₁ is absent. Considering this in view of the methoddemonstrated with reference to FIG. 7 above, the estimation 702 provides|h₀| and |h₁|, and the determination 704 checks whether ∥h₀|−|h₁∥>E₁,and if so, s₁ transmitted over h₁ is disregarded 706 and s₁ isconsidered transmitted in the detection 708 when s₀ is determined not tobe present according to what is suggested above.

The approach described above can be viewed as very simple, but it isalso clear that it may be sub-optimum. To fully explore the potentialdiversity gain it is desirable to use both channels h₀ and h₁ for makingthe decision, but in a way that it is taken into account that thechannel quality is different. To do this, consider the followingapproach, again assuming Manchester coded OOK and that |h₀| and |h₁| areknown, e.g. by the estimation. Making the assumption that the noise isGaussian with variance σ², the probability density function (pdf) forthe decision metric in case the decision metric is obtained byintegrating the received signal, i.e., first half minus second half. Incase s₁ is sent, the result is

${f_{1}(r)} = {\frac{1}{\sqrt{2\pi\sigma^{2}}}e^{- \frac{{({r - {s_{1}{h_{1}}}})}^{2}}{2\sigma^{2}}}}$

If instead so is sent, the corresponding pdf becomes

${f_{0}(r)} = {\frac{1}{\sqrt{2\pi\sigma^{2}}}e^{- \frac{{({r + {s_{0}{h_{0}}}})}^{2}}{2\sigma^{2}}}}$

The log-likelihood can then be formed using f₁(r) and f₀(r) and if wenormalize s₁=s₀=1 the result is that one should decide that s₁ was sentif

$r > \frac{{h_{1}} - {h_{0}}}{2}$

It can be noted that in case the channels are equally good, the decisionboundary becomes 0 and in case one would adopt the simplified approachof only using the better of the channels, the decision boundary becomeshalf of the expected value.

FIG. 8 is a flow chart schematically illustrating methods of a receiveraccording to embodiments. The method comprises receiving 800 a signalrepresenting a sequence of bits keyed and transmitted e.g. asdemonstrated above. That is, the signal is multi-layered, wherein eachlayer may, due to the transmit diversity, have a channel which differsfrom the other layers. The channels are estimated 802 for the respectivelayers. It can then be determined 804 whether one of the channels havesignificantly lower channel quality. For example, channel gain such as|h₁|, |h₂|, etc. may be considered as a quality metric. A determinedquality metric for the respective channels may be compared and acomparison result may in turn be compared with a first thresholddefining whether one channel has significantly lower quality. If nochannel has significantly lower quality, the received signal is detected814 using legacy procedures. However, if one channel has significantlylower quality, it is determined 806 whether the best channel gain isabove a second threshold. If it is, detection 810 is performed usingreceived amplitudes or energy as demonstrated above. If not, there islikely no benefit with the special detection 810, and the signal isdetected 814 in the conventional way.

FIG. 9 schematically illustrates a transmitter 900 according to anexample, which transmitter 900 is arranged to transmit binaryinformation which uses the binary amplitude shift keying demonstratedabove with reference to the different embodiments. Information symbols902 are represented by a signal which is transmitted, including themulti-layered transmissions 904 demonstrated above, through two or moreantennas. In FIG. 9, the multi-layered transmissions 904 are the exampleas of what is demonstrated with reference to FIG. 6, but anymulti-layered transmissions demonstrated herein are of course equallyfeasible. The transmitter 900 typically is a part of an access pointforming a network node of e.g. a radio access network.

FIG. 10 schematically illustrates a receiver 1000 according to anembodiment, which receiver 1000 is arranged to receive binaryinformation which uses the binary amplitude shift keying demonstratedabove with reference to the different embodiments. Information symbols1002 are represented by a received signal including the multi-layeredtransmissions 1004 demonstrated above. In FIG. 10, the multi-layeredtransmissions 1004 are the example as of what is demonstrated withreference to FIG. 8, but any multi-layered transmissions demonstratedherein are of course equally feasible. The receiver 1000 is typically apart of a wireless communication device comprising a WUR and a mainreceiver or transceiver for PCR transmissions and arranged to beactivated by the WUR. The receiver 1000 is then part of the WURreceiver. In FIG. 10, the receiver would typically only have oneantenna, but depending on which precoder is used by the transmitter thereceiver will effectively experience different channels. In FIG. 10 thisis illustrated by a dashed antenna representing that one of theinformation bits is estimated using a different precoder than the otherbit.

FIG. 11 is a block diagram schematically illustrating a communicationdevice 1100 according to an embodiment. The communication devicecomprises an antenna arrangement 1102, a receiver arrangement 1104connected to the antenna arrangement 1102, a transmitter arrangement1106 connected to the antenna arrangement 1102, a processing element1108 which may comprise one or more circuits, one or more inputinterfaces 1110 and one or more output interfaces 1112. The interfaces1110, 1112 can be operator interfaces and/or signal interfaces, e.g.electrical or optical. The communication device 1100 may be arranged tooperate in a cellular communication network.

The communication device may for example be arranged such that thetransmitter arrangement 1106 comprises a transmitter as demonstratedwith reference to FIG. 9. In particular, by the processing element 1108being arranged to perform the examples demonstrated above, thecommunication device 1100 is capable of transmitting a signal asdemonstrated above. The communication device may according toembodiments be an access point providing a WUS as demonstrated above.

The communication device may according to examples be arranged such thatthe receiver arrangement 1104 comprises a receiver as demonstrated withreference to FIG. 10, forming a WUR arranged to provide activationsignals upon reception of a proper WUS to a PCR receiver of the receiverarrangement 1104. The receiver arrangement 1104 is here to be regardedas either a single receiver used for both the signal demonstrated above,e.g. wake-up signal, paging signal, control signal, etc., and for othertraffic, e.g. associated with a cellular or wireless local area network,or as a receiver arrangement comprising one receiver arranged fortraffic associated with e.g. a cellular or wireless local area network,and another receiver arranged and dedicated to receive the signaldemonstrated above. In particular, the communication device 1100according to an embodiment is capable of receiving a multi-layeredsignal as demonstrated above. The communication device according to oneembodiment may be a wireless communication device arranged for leanoperation by being capable of receiving a WUS as demonstrated above.

The processing element 1108 can for some of the embodiments and examplesalso fulfil a multitude of tasks, ranging from signal processing toenable reception and transmission since it is connected to the receiver1104 and transmitter 1106, executing applications, controlling theinterfaces 1110, 1112, etc.

The methods according to the present disclosure are suitable forimplementation with aid of processing means, such as computers and/orprocessors, especially for the case where the processing element 1108demonstrated above comprises a processor handling the estimation of thechannels and selection of detection method. Therefore, there is providedcomputer programs, comprising instructions arranged to cause theprocessing means, processor, or computer to perform the steps of any ofthe methods according to any of the embodiments described with referenceto FIGS. 1 to 10. The computer programs preferably comprise program codewhich is stored on a computer readable medium 1200, as illustrated inFIG. 12, which can be loaded and executed by a processing means,processor, or computer 1202 to cause it to perform the methods,respectively, according to embodiments of the present disclosure,preferably as any of the embodiments described with reference to FIGS. 1to 10. The computer 1202 and computer program product 1200 can bearranged to execute the program code sequentially, where actions of theany of the methods are performed stepwise, or be made to perform theactions on a real-time basis. The processing means, processor, orcomputer 1202 is preferably what normally is referred to as an embeddedsystem. Thus, the depicted computer readable medium 1200 and computer1202 in FIG. 12 should be construed to be for illustrative purposes onlyto provide understanding of the principle, and not to be construed asany direct illustration of the elements.

A transmitter suitable to provide the transmissions to be received bythe above demonstrated receiver, corresponding method, andimplementations thereof may according to examples provide featuresaccording to the following items:

1. A method of transmitting an amplitude shift keyed signal using amulti-layered transmission over a plurality of transmit antennas withdifferent precoding of different symbols for the respective layers, themethod comprising

obtaining a sequence of bits to be conveyed;

keying the sequence of bits to a signal;

precoding the signal to respective layer; and

transmitting the precoded signal.

2. The method of item 1, wherein the keying of the bits comprisesManchester coding the bits.

3. The method of item 1 or 2, wherein the precoding to respective layeris performed on a line for respective layer of a precoding matrix.

4. The method of item 3, comprising selecting the precoding matrix froma set of matrices.

5. The method of item 4, wherein the selecting of the precoding matrixis based on a mapping between precoding matrix and addressed receiver ofthe transmission.

6. The method of item 4 or 5, wherein multiple signals are multiplexedin the transmission, and the selecting of the precoding matrix is basedon a mapping between respective multiplexed signal and respectiveprecoding matrix.

7. The method of anyone of items 4 to 6, wherein for each matrix of theset of matrices, two of the lines are the same for all matrices and arearranged to be used for pilot symbols.

8. The method of any one of items 1 to 7, wherein each layer istransmitted by a respective antenna or set of antennas.

9. The method of any one of items 1 to 8, wherein the precoding isperformed by

assigning a first part of the signal to a first layer; and

assigning a second part of the signal to a second layer.

10. The method of item 9, wherein the second part of the signal is arepeated replica of the first part of the signal.

11. The method of any one of items 1 to 8, wherein the precoding isperformed by

assigning symbols for a first bit value to a first layer; and

assigning symbols for a second bit value to a second layer.

12. The method of any one of items 1 to 11, wherein at least a part ofthe sequence comprises pilot bits.

13. The method of item 12, wherein the part of the signal correspondingto the pilot bits are precoded according to a predetermined precodingpattern.

14. A computer program comprising instructions which, when executed on aprocessor of a transmitter, causes the transmitter to perform the methodaccording to any of claims 1 to 13.

15. A transmitter for transmitting an amplitude shift keyed signalprovided over a multi-layered transmission over a plurality of transmitantennas with different precoding of different symbols for therespective layers, arranged to

obtain a sequence of bits to be conveyed;

key the sequence of bits to a signal;

precode the signal to respective layer; and

transmit the precoded signal.

16. The transmitter of item 15, wherein keying of the bits comprisesManchester coding of the bits.

17. The transmitter of item 15 or 16, wherein, for respective layer, aline of a precoding matrix determines the precoding for respectivelayer.

18. The transmitter of item 17, arranged to select the precoding matrixfrom a set of matrices.

19. The transmitter of item 18, wherein the selection of the precodingmatrix is based on a mapping between precoding matrix and addressedreceiver of the transmission.

20. The transmitter of item 18 or 19, wherein multiple signals aremultiplexed in the transmission, and the selection of the precodingmatrix is based on a mapping between respective multiplexed signal andrespective precoding matrix.

21. The transmitter of anyone of items 18 to 20, wherein for each matrixof the set of matrices, two of the lines are the same for all matricesand are arranged to be used for pilot symbols.

22. The transmitter of any one of items 15 to 21, comprising themultiple transmit antennas, wherein each layer is transmitted by arespective antenna or set of antennas of the plurality of transmitantennas.

23. The transmitter of any one of items 15 to 22, wherein the precodingincludes that

a first part of the signal is assigned to a first layer; and

a second part of the signal is assigned to a second layer.

24. The transmitter of item 23, wherein the second part of the signal isa repeated replica of the first part of the signal.

25. The transmitter of any one of items 15 to 22, arranged to, for theprecoding,

assign symbols for a first bit value to a first layer; and

assign symbols for a second bit value to a second layer.

26. The transmitter of any one of items 15 to 25, wherein at least apart of the sequence comprises pilot bits.

27. The transmitter of item 26, wherein the part of the signalcorresponding to the pilot bits are precoded according to apredetermined precoding pattern.

Other transmitters and transmit methods may be provided which obtain thesimilar transmissions, which transmissions are beneficial to receivewith the reception approaches demonstrated above.

To demonstrate the performance of the two main schemes demonstratedabove, the following simulations consider communication over a flatRayleigh fading channel with each of the two transmit antennas subjectto independent fading. The channel realizations are fixed for theduration of one packet. A 128-bit packets transmitted over a 4 MHzchannel is considered. The values of |h₁| and |h₂| are estimated blindlyfrom the received signals.

FIG. 13 shows the packet error rate (PER) with uncoded transmission as afunction of the SNR over a 20 MHz channel using a rate of 62.5 kbit/s,where each symbol is encoded as either ON OFF ON OFF or OFF ON OFF ON.With the first scheme, employing the approach demonstrated e.g. withreference to FIGS. 5 and 7 above and marked “Proposed 1” in FIG. 13,different antennas are used for the first and second half of a symboltransmission. With the second scheme, employing the approachdemonstrated e.g. with reference to FIGS. 6 and 8 above and marked“Proposed 2” in FIG. 13, “0” and “1” are transmitted using differentantennas.

FIG. 13 shows that “Proposed 1” gives a lower PER than the baselinescheme, i.e. legacy approach, for any SNR and the slope is also steeper,which proves that the scheme successfully achieves transmit diversityagainst channel fading. “Proposed 2” achieves the same slope as“Proposed 1” and is preferable over the baseline scheme in the intervalof interest, with PER of 10 and lower. However, at low signal-to-noiseratios (SNRs), “Proposed 2” gives higher PERs than the baseline schemesince the amplitudes of |h₁| and |h₂| are small and therefore thebenefits of the scheme can seldom be utilized, i.e., the secondthreshold in comparison 806 of FIG. 8 is seldom satisfied.

Next, a rate of 125 kbit/s is considered, where each symbol is encodedas either ON OFF or OFF ON. In this case, “Proposed 1” cannot be usedand comparison is only made for the baseline scheme with “Proposed 2”.FIG. 14 shows the PER as a function of the SNR over a 20 MHz channel.Similar to FIG. 13, in the interval of interest, with PER of 10⁻¹ anddownwards, “Proposed 2” outperforms the baseline scheme. Note that thecurve of the proposed approach has a steeper slope, which proves that itsuccessfully achieves transmit diversity against channel fading.

Further features may be employed with the approaches demonstrated above.In one embodiment, the matrix W is used to define lobes withpre-determined spatial directions. Instead of sending “0” and “1”through two of the antennas, the corresponding signals are sent in thepre-defined lobes. The receiver can then estimate the strength and therelative phase difference between the lobes.

In one example, more than two antennas are used for precoding. Forexample, in the schemes “Proposed 1” and “Proposed 2”, with twoantennas, antenna 1 and antenna 2 are interchangeably used for thetransmission. With more than two antennas, at each time instant the twoantennas that are switched between may be selected according to somepre-determined pattern. Alternatively, the notation of “antenna 1” and“antenna 2” can be interpreted as virtual antennas created by twodifferent precoding vectors, which are transmitted using more than twoantennas.

In one example, multiple WUS are multiplexed, e.g. for addressingdifferent receivers. Different precoding patterns can be assigned to thedifferent WUSs.

In one example, additional side-information regarding a user, e.g.,regarding previously successful transmission of a WUS, spatialdirections, or other channel statistics, is utilized when selecting theprecoding for said user. This information can also be used whenselecting precoding for other users, to avoid interference.

In the description above, it is assumed in the given examples that thereceiver is based on an envelope detector, since this is commonly used.The disclosure does not rely on this being the case, however. Anyreceiver which is suitable for receiving OOK could benefit from theabove described embodiments. In one embodiment, instead of using theenvelope of the signal, the power of the signal may be used. If thereceiver is implemented in the digital domain, envelope or power willhave essentially the same implementation complexity. If the receiver toa larger extent is implemented in the analog domain, it is more of adesign choice whether an envelope or power detector is preferred.

1-16. (canceled)
 17. A method of a receiver for receiving an amplitudeshift keyed signal provided over a multi-layered transmission from aplurality of antennas with different precoding of different symbols forthe respective layers, the method comprising receiving a sequence ofsignal values of the signal; estimating, from the sequence of signalvalues, channels for the respective layers; and selecting one of aplurality of detection methods based on a difference in quality betweenthe estimated channels.
 18. The method of claim 17, the receivingcomprising estimating, via an envelope detector, the signal values asenergies of the signal, and the estimating of the channels comprisesestimating channel gain by selecting at least one signal valueassociated with a respective layer and performing the estimation of thechannels from the selected at least one signal value for the respectivelayer.
 19. The method of claim 18, wherein the estimating of thechannels comprises estimating phase differences between the channels.20. The method of claim 17, wherein the signal is sent as a repeatedsymbol sequence for each layer, and the selecting of the detectionmethod comprises: determining whether one of the estimated channels hasa significantly lower quality than another of the estimated channels;wherein, if the one of the estimated channels has the significantlylower quality, the selected detection method comprises omitting a partof the signal with the lower quality and decoding the rest of thesignal, and if there is no estimated channel having the significantlylower quality, the selected detection method comprises decoding thewhole signal.
 21. The method of claim 20, wherein the determiningwhether the one of the estimated channels has significantly lowerquality than another one of the estimated channels comprises determiningwhether difference between the respective channel gains is greater thana first threshold.
 22. The method of claim 17, wherein the selecting ofthe detection method comprises: determining whether one of the estimatedchannels has a significantly lower quality than another of the estimatedchannels; and determining whether a largest of channel gain of one ofthe estimated channels and another of the estimated channels is greaterthan a second threshold; wherein, if the one of the estimated channelshas the significantly lower quality and the largest channel gain of thechannel gains is greater than the second threshold, the selecteddetection method comprises omitting a part of the signal with the lowerquality and decoding the rest of the signal, and, otherwise, theselected detection method comprises decoding the whole signal.
 23. Themethod of claim 18, wherein the signal is sent as a repeated symbolsequence for each layer, and the selecting of the detection methodcomprises: determining whether a difference between channel gain of oneof the estimated channels and another of the estimated channels isgreater than a first threshold; and determining whether a largest ofchannel gain of one of the estimated channels and another of theestimated channels is greater than a second threshold; wherein, if theone of the estimated channels has the difference in channel gainsgreater than the first threshold and the largest channel gain is greaterthan the second threshold, the selected detection method comprisesomitting a part of the signal with the lower quality and decoding therest of the signal, and, otherwise, the selected detection methodcomprises decoding the whole signal.
 24. A communication devicecomprising: receiver circuitry; and processing circuitry; wherein, withrespect to an amplitude shift keyed signal provided over a multi-layeredtransmission with different precoding for the respective layers andreceived via the receiver circuitry, the processing circuitry isconfigured to: receive a sequence of signal values of the amplitudeshift keyed signal; estimate, from the sequence of signal values,channels for the respective layers; and select one of a plurality ofdetection methods based on a difference in quality between the estimatedchannels.
 25. A receiver that is configured to, with respect toreceiving an amplitude shift keyed signal provided over a multi-layeredtransmission with different precoding for the respective layers: receivea sequence of signal values of the amplitude shift keyed signal;estimate, from the sequence of signal values, channels for therespective layers; and select one of a plurality of detection methodsbased on a difference in quality between the estimated channels.
 26. Thereceiver of claim 25, comprising an envelope detector arranged toestimate the signal values as energies of the signal, wherein theestimation of the channels comprises estimation of channel gain byselecting at least one signal value associated with a respective layerand performing the estimation of the channels from the selected at leastone signal value for the respective layer.
 27. The receiver of claim 26,wherein the estimation of the channels comprises estimation of phasedifferences between the channels.
 28. The receiver of claim 25, whereinthe amplitude shift keyed signal is sent as a repeated symbol sequencefor each layer, and wherein to select the detection method the receiveris configured to: determine whether one of the estimated channels has asignificantly lower quality than another of the estimated channels;wherein, if the one of the estimated channels has the significantlylower quality, the selected detection method comprises omitting a partof the signal with the lower quality and decoding the rest of thesignal, and, if there is no estimated channel having the significantlylower quality, the selected detection method comprises decoding thewhole signal.
 29. The receiver of claim 28, wherein the determination ofwhether one of the estimated channels has significantly lower qualitythan another of the estimated channels comprises a determination ofwhether differences between channel gains is greater than a firstthreshold.
 30. The receiver of claim 25, wherein the receiver isarranged to determine whether one of the estimated channels has asignificantly lower quality than another of the estimated channels, anddetermine whether a largest of channel gain of one of the estimatedchannels and another of the estimated channels is greater than a secondthreshold, and, wherein, if the one of the estimated channels has thesignificantly lower quality and the largest channel gain of the channelgains is greater than the second threshold, the selected detectionmethod comprises omitting a part of the signal with the lower qualityand decoding the rest of the signal, and, otherwise, the selecteddetection method comprises decoding the whole signal.
 31. The receiverof claim 26, wherein the signal is sent as a repeated symbol sequencefor each layer, and wherein the receiver is arranged to determinewhether a difference between channel gain of one of the estimatedchannels and another of the estimated channels is greater than a firstthreshold, and determine whether a largest of channel gain of one of theestimated channels and another of the estimated channels is greater thana second threshold, and wherein, if the one of the estimated channelshas the difference in channel gains greater than the first threshold andthe largest channel gain is greater than the second threshold, theselected detection method comprises omitting a part of the signal withthe lower quality and decoding the rest of the signal, and, otherwise,the selected detection method comprises decoding the whole signal. 32.The receiver of claim 25, wherein the receiver is arranged as a wake-upreceiver that controls on and off states of a main transceiver independence on the amplitude shift keyed signal, and wherein the wake-upreceiver is co-located with or integrated into the main transceiver.